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Hollow Cathodes 311 300- to 600-V range. Once the discharge is ignited, the keeper current is limited by the power supply and the voltage falls to a low value below the discharge voltage. Once ignited, hollow cathodes are well known to operate in distinct discharge “modes.” In ion thrusters, the hollow cathode discharge operation has been historically characterized as having a quiescent “spot mode” with a broadly optimum gas flow at a given current, and a noisy “plume mode” with the gas flow below the level at which the spot mode is obtained [44,45]. The spot mode, seen in Fig. 6-28(a), is visually observed as manifesting a ball or “spot” of plasma just downstream of the cathode orifice with little visual glow from the downstream plasma at low currents and a slowly expanding plasma column extending from the spot into the thruster discharge chamber at higher currents. The plume mode is seen visually as a widely diverging plasma cone extending from the cathode, often filling the vacuum chamber with diffuse plasma and little or no spot or ball of plasma in the cathode/keeper orifice. There is a continuous transition between these modes, which is sometime separately identified as a transition mode [3]. A less well-known third mode, sometimes called a “stream mode,” occurs at high gas flows well above the optimum for the spot mode. In this stream mode, shown just starting in Fig. 6-28(b), the plasma spot is pushed well down stream of the cathode/keeper orifice, and a dark space between the cathode or keeper electrode and the spot is usually observed. In this case, the plasma expands and disperses faster than in the normal spot mode. Very high cathode flow rates tend to suppress the discharge voltage, which adversely affects the ionization rate and discharge performance in discharge cathodes in ion thrusters. However, higher flow rates tend to reduce the coupling voltage in both Hall and ion thruster neutralizer cathodes, which can improve the performance. The hollow cathode discharge modes have been examined in detail due to the observed increases in the keeper or coupling voltages in the plume mode [44,45,64,69,70] and increases in keeper wear [53,69]. At flow rates near the optimum for the spot mode, thermionic hollow cathodes can produce quiescent discharges [46,64]. In neutralizer cathodes, transition to plume mode occurs when too low a gas flow rate and/or keeper current is provided for the desired emission current. Plume mode transition is usually detected by increases in the oscillation of the keeper voltage or in the magnitude of the coupling voltage. For example, plume mode onset is defined in the NSTAR neutralizer when the keeper voltage oscillation exceeds 5 V [71]. In discharge cathodes, transition to plume mode also occurs for too low a propellant flow at a given emission current (or too high a discharge current for a given flow), and is usually detected by increases in the discharge voltage oscillations. Transition to plume mode usually occurs at higher emission current densities (related to the size ofPDF Image | Fundamentals of Electric Propulsion: Ion and Hall Thrusters
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